1. Field of the Invention
The present invention relates to a subscriber circuit, provided with a feeding circuit and a switching circuit group, for controlling feeding to a terminal of a telephone subscriber, and more particularly, it relates to a subscriber circuit provided with a signal monitoring function for monitoring a signal transferred through a subscriber line.
2. Description of the Related Art
Generally a voice signal for telephone call has been transferred to a telephone subscriber line, as well as a signal including predetermined information such as detection of the hook with the signal superimposed on the voice signal.
In the conventional technique, a signal monitor for the exclusive purpose is provided in order to monitor this kind of signal transferred through a subscriber line. As this kind of the conventional signal monitor, there is, for example, a device disclosed in Japanese Patent Publication Laid-Open (Kokai) No. Showa 59-128855. The conventional signal monitor circuit disclosed in the same publication comprises a high impedance dc buffer connected to a subscriber line at high input impedance, a hook detecting device for detecting the state of a hook of a subscriber line through the output of the high impedance dc buffer, and a pilot sound detecting device for monitoring a pilot sound in reply to the hook detecting device.
The conventional signal monitor is designed to monitor a signal transferred from a terminal through a subscriber line, and therefore, it is able to monitor a signal only when the terminal thereof is connected to a feeding circuit and the feeding circuit is feeding the power. When the terminal side is on the hook and the subscriber line stops feeding the current of a call, or when the feeding circuit is released from the subscriber line, it cannot monitor a signal.
The above-mentioned conventional signal monitoring technique, however, must be provided with a device for the exclusive use for monitoring a signal, and therefore, the system is increased in size and it becomes complicated, which is not economical.
This is because the pilot sound monitor owes the signal monitor of a subscriber line and the high input impedance dc buffer and the hook detecting device owe the detection of the hook of a subscriber line.
In the conventional signal monitor, the high input impedance dc buffer, that is one of the components, needs to have high input impedance, which produces such ill effects that the level of a call is deviated, echoes increase, and the like.
This is because the terminal impedance in a subscriber line is deviated. Namely, since a telephone signal terminates at constant terminal impedance at the time of a telephone call, it produces little effect on the call logically if terminal impedance<<dc buffer. Actually, since even the high-input impedance has limited value, it is impossible to be completely free from ill effect, but deviation of the terminal impedance in the subscriber line will produce some effect on the call.
The conventional signal monitor cannot monitor a signal of a subscriber line when the feeding circuit stops feeding and the feeding circuit is cut off from the subscriber line. Therefore, for example, when reversing the feeding polarity of a subscriber, since the feeding circuit is stopped and the feeding circuit is once cut off from the subscriber line, the signal monitor becomes impossible.